An MRI apparatus is designed to generate a magnetic resonance imaging phenomenon to a nuclear spin in an object to be examined by applying a radio-frequency magnetic field on the object laid in a static magnetic field, measure a nuclear magnetic resonance signal generated inside the object, and process the signal to produce an image.
Imaging techniques of the MRI apparatus include the spin echo method (SE method), a gradient echo method (GE method), an echo planer method (EPI method) based on the SE method or GE method which are used for obtaining a morphological image and a blood flow image of the object, and also include a method used for obtaining an image reflecting strength of Brownian motion of water molecule in the object's tissue. The method for obtaining an image reflecting strength of Brownian motion of water molecule in the object's tissue is referred to as a diffusion weighted imaging method.
The diffusion weighted imaging method suppresses a magnetic resonance signal from a fluent substance inside the object having diffluence such as a water molecule, and thus emphasizes a signal from a portion where the molecule does not move. It is reported that the method is useful for diagnosis of brain infarction, particularly of newly developed brain infarction.
Diffusion weighted imaging based on SE-EPI will be described here. In the SE-EPI, a slicing gradient magnetic field and a 90° RF pulse are first applied to excite a nuclear spin in a slice of the object. Then, the slicing gradient magnetic field is then applied again and a 180° RF pulse is applied to invert the above excited nuclear spin. After applying an offset gradient magnetic field in a phase encoding direction, a readout gradient magnetic field and a phase encoding gradient magnetic field are applied several times at a predetermined time interval, and a plurality of magnetic resonance signals (echo signals) are measured. To perform the diffusion weighted imaging, a pair of gradient magnetic field pulses (diffusion weighted gradient magnetic field pulse) is applied in a desired gradient magnetic field direction before and after the application of 180° RF pulse. By applying the diffusion weighted gradient magnetic field pulse, the echo signal to be measured is provided with diffusion information. The diffusion weighted gradient magnetic field may be applied in one direction, two directions, or three-directions. Thus measured echo signal is reconstructed and a diffusion weighted image is obtained.
By the way, the EPI pulse sequence includes a single-shot EPI in which the nuclear spin is excited once to measure echo signals by the number required for image reconstruction, and a multi-shot EPI in which the nuclear spin is excited several times to measure echo signals by the number necessary for image reconstruction.
It is known that in the MRI apparatus, when the object moves during measurement of a plurality of echo signals, a phase difference occurs among signals generated from the same portion before and after the movement. The same is true of the multi-shot EPI method. Accordingly, when the diffusion weighted imaging is performed using the multi-shot EPI, if the object's body movement is different at each shot, artifacts (hereinafter referred to as motion artifact) are generated in the reconstructed image due to the phase difference occurring among the echo signals.
As a method of correcting the motion artifacts, a motion correction method using navigation echo (hereinafter referred to as navigated motion correction method) is known. In the navigated motion correction method based on the multi-shot SE-EPI, a 90° RF pulse is applied to excite a nuclear spin, a 180° RF pulse is next applied, a gradient magnetic field having a predetermined strength and application time is then applied in a predetermined direction along with a readout gradient magnetic field, and an echo signal for monitoring body movement (navigation echo) is measured. After the navigation echo is measured, a phase encoding gradient magnetic field and a readout gradient magnetic field are repeatedly applied and a plurality of echo signals for imaging (actual measurement) are measured. When a first shot of the above pulse sequence is completed, a second shot, a third shot, . . . an nth shot, . . . and an Nth shot are sequentially performed in a similar manner thereto.
Navigation echo nav(n) and actual echo signals echo(n,m) are thus measured by sequentially executing the first to Nth shots. Here, n represents a shot number of the pulse sequence where 1≦n≦N, and m represents a measurement number of echo signal in each shot where 1≦m≦M.
The motion correction of echo signal in the actual measurement is done using thus measured navigation echoes. In the first step of the motion correction, one navigation echo is selected as reference navigation echo nav0 from among above navigation echoes nav(n), ordinarily, the navigation echo of the first shot is selected. In the second step, reference navigation echo nav0 is one-dimensionally Fourier-transformed in the readout direction. Next, in Step 3, navigation echoes other than the reference navigation echo nav0 are also one-dimensionally Fourier-transformed in the readout direction. Furthermore, in Step 4, a phase of data array of the reference navigation echo subjected to Fourier transformation is calculated, phases of data array of other navigation echoes subjected to Fourier transformation are also calculated, and phase difference θ(n) between the phase of the reference navigation echo then that of other navigation echoes is calculated.
Because calculated phase difference θ(n) is generated due to a difference between a motion component occurring in measurement of the reference navigation echo and each of those in measurement of other reference navigation echoes, a correction of motion component included in actual echo signal echo (n,m) is executed using phase difference θ(n) in Step 5.
By executing the above Steps 1 to 5, artifacts caused by the object's movement and included in the actual echo signals obtained by executing the pulse sequence N times are eliminated.
The methods of motion correction using the navigation echo, particularly known as techniques of stably obtaining an image in which motion artifacts are reduced, include a diffusion weighted imaging mentioned in Japanese Unexamined Patent Publication Hei.09-299345 (hereinafter referred to as conventional example 1) and one mentioned in Japanese Unexamined Patent Publication Hei.11-128202 (hereinafter referred to as conventional example 2). In those conventional examples, it is judged using the phase difference whether each of actual echo signals can be corrected or not, and those which cannot be corrected are not used in image reconstruction.
In the diffusion weighted imaging method according to conventional example 1, an average value of phase difference among the plurality of navigation echoes is used as a reference value, and a phase difference between the reference value and a phase value of each navigation echo is calculated. It is judged that the navigated motion correction cannot be performed on a group of actual echo signal acquired in correspondence with a navigation echo having a phase difference larger than an acceptable value, and that navigation echo and actual echo signal are re-collected. In this manner, all actual echo signals for producing an image are made into data which can be properly corrected with the navigated motion correction. Thus, an image with reduced motion artifacts is obtained.
On the other hand, in the diffusion weighted imaging method of conventional example 2, the reference value is an integral value or a peak value calculated from a predetermined navigation echo, or an area or a peak value of a projection pattern obtained by one-dimensionally Fourier-transforming the predetermined navigation echo. Alternatively, an average value of the integral value or the peak value calculated from the predetermined navigation echo, or that of the area or the peak value of a projection pattern obtained by one-dimensionally Fourier-transforming the predetermined navigation echo is used. A value calculated from each navigation echo is compared with the reference value, and it is judged that the navigated motion correction cannot be performed on a group of actual echo signals acquired in correspondence with a navigation echo having a comparison result exceeding an acceptable value. That group of actual echo signals is replaced with another echo signal group or re-collected. In this manner, all of measured echo signals for producing an image are made into data which can be properly corrected with the navigated motion correction. Thus, an image with reduced motion artifacts is obtained.
However, there is a case where sufficient motion correction cannot be performed even with those improved navigated motion correction methods. For example, it is reported in “Analysis and Correction of Motion Artifacts in Diffusion Weighted Imaging┘ Adam W. Anderson, John C. Gore, MRM 32: 379–387 (1994)” that when the head being an imaging portion of the object moves backward and forward with revolution due to the object's respiration movement during the application of diffusion gradient magnetic field, a linear phase gradient occurs in the phase encoding direction of the imaging area, and so correction effect of the navigated motion correction cannot be expected.
The reason is that the phase value of a navigation echo measured in a state where phase errors vary with respect to the phase encoding direction of the imaging area is an integral value of phases of nuclear spins existing in the phase encoding direction of the imaging area, and so a variation component of phase errors in the phase encoding direction cannot be extracted from that value.
Accordingly, in the both diffusion weighted imaging methods according to conventional example 1 and 2, it is impossible to correctly judge a degree of movement generating the linear phase gradient in the phase encoding direction in the imaging during measurement. Therefore, it is difficult to stably reduce the motion artifacts.
The first object of the present invention is to provide a magnetic resonance imaging method enabling accurate judgment of phase errors including those in a phase encoding direction in the diffusion weighted imaging.
Further, the second object of the invention is to provide an MRI apparatus which can obtain an image in which artifacts generated due to the object's movement are reduced.